1. Field of the Invention
The present invention relates to a resistance-change-type fuse circuit which causes a current to flow through a polysilicon fuse to cause a change in resistance.
2. Related Art
In recent years, system LSIs in which a large number of memories such as SRAMs and DRAMs are mixedly mounted on a chip have come into wide use. Such on-chip SRAMs and DRAMs have become finer and the capacity of such memories is being increased. It is, therefore, difficult to achieve 100% yield in producing such memories.
A method has therefore been used in which redundant cells are incorporated and cells determined as defectives by a test are replaced with the redundant cells to improve the yield. Information on which cells are replaced with redundant cells is ordinarily programmed in one time programmable (OTP) devices capable of being programmed only once.
A process management method has also been practiced in which ID information specific to a chip (chip ID) including a name of a factory where the chip has been manufactured, a line number, a lot number, a wafer number and X- and Y-coordinates on the wafer is programmed in OTP elements, and in which when the chip is sent back because of occurrence of a fault with the chip on the market, the chip ID is read out to check whether or not there has been any abnormality in the manufacturing process.
OTP elements in which redundancy information for the above-mentioned on-chip SRAMs and DRAMs and chip ID information are programmed include optically programmable fuses which are programmed by using light such as laser light, and electrically programmable fuses.
Optically programmable fuses have an advantage in that fabrication processes are easy and a ratio of the resistance of an element already programmed to the resistance of an intact element is large. Therefore, the optically programmable fuses have been widely used. However, optically programmable fuses have a drawback in that it is impossible to program it after the chip is assembled into a package. On the other hand, electrically programmable fuses have an advantage in that it is possible to program it after packaging the chip. Therefore, recently, the electrically programmable fuses have been widely used (see U.S. Pat. No. 6,525,397).
As one of electrically programmable fuses, a polysilicon resistance-change-type fuse (hereinafter referred to as “polysilicon fuse”) exists which is programmed in such a manner that a large current is caused to flow through polysilicon in salicide form used as a gate material, and electromigration phenomenon occurs in which metal atoms in the salicide portion on polysilicon is moved in the same direction as the electron, thereby increasing the resistance of polysilicon for programming.
However, the conventional polysilicon resistance-change-type fuse has problems described below.    1) The current flowing through the polysilicon fuse is controlled through a voltage VPGM applied to one end of the fuse and a voltage VGATE applied to a gate of a transistor (programming transistor) for switching control as to whether or not the current is to be caused to flow through the fuse. The optimum values of the voltages VPGM and VGATE depend on the characteristics of the polysilicon fuse and the programming transistor.
When the characteristics of the programming transistor are changed due to fluctuation in manufacturing process, there is a possibility of changing the drain current Ids of the transistor and deviating from the desired current range even if a constant voltage is applied to the gate. Deviation from the desired current range may lead to failure to program the polysilicon fuse. Possible causes of programming failure include a situation where the current is so small that the resistance value of the fuse does not change, and a situation where the current is so large that the fuse has an excessively high resistance.    2) With fluctuation in the manufacturing process, the thickness and width of the polysilicon fuse change slightly to vary the resistance value of the fuse before programming. If the resistance value of the polysilicon fuse before programming varies, there is a possibility in which the drain voltage Vds of the programming transistor changes so that the drain current Ids of the programming transistor fluctuates and deviates from the desired current range. Also in this case, there is a possibility of failure to program the polysilicon fuse.    3) An arrangement may be adopted in which several combinations of a trial-cutting polysilicon fuse and a programming transistor are prepared on one chip and voltages for the combinations are successively applied to cause a current to flow. However, the operation to measure the current with an ordinary semiconductor tester while changing the voltage requires a long time to perform. It is necessary to obtain the optimum value by repeating the operation several times. A further increased operation time is required for this, and the testing efficiency is reduced. As a result, the testing cost increases.    4) The act to program the polysilicon fuse is irreversible. Once a current is caused to flow through the polysilicon fuse, the fuse cannot be again programmed. In a case where the voltages VPGM and VGATE are changed to find the optimum combination of the voltages VPGM and VGATE, there is a need to prepare the number of combinations of the trial-cutting polysilicon fuse and the programming NMOS transistor in correspondence with the number of combinations of VPGM and VGATE. This means an increase in chip area and, hence, an increase in chip cost.